1. Technical Field
The present invention relates to electromagnetic radiation shielding for bulk cable.
2. Description of the Related Art
It has long been known to coat current carrying cables with an insulative material to isolate electric current from the environment. For example, such insulation protects undesirable shorting or grounding of the wire cable. Where it is desired to have an electrical connection, the insulation can be stripped from the wire. While such coatings are effective as electric insulators, they are not effective as shields of electromagnetic energy.
Electronic equipment has been shielded by totally enclosing the equipment in a metal "container" or housing that is continuously conductive. The shielding of wires and cables has been accomplished in a similar manner by maintaining the wires and cables, with the electrically insulating coating, within metal pipes, the metal pipe providing the necessary shield. Such shields are extremely cumbersome since they are rigid and provide no flexibility to the system.
A more flexible shielding conduit can be made of woven metal braid, stripped metal formed into spiral bellows or some other spiral that allows interlocking of adjacent strips. Often times a foil layer is placed between the conductors and shield for a temperature barrier and shield enhancement.
The multiple steps and tedious process of weaving the braid makes this method for creating a shielded cable both cumbersome and expensive. Also, although such conduits are more flexible than metal pipes, they are not sufficiently flexible for many applications, and they suffer the additional disadvantage of providing inferior shields of electromagnetic energy due to the leakage of electromagnetic energy through the braids, joints, and other openings which are found in these types of conduits. This loss of shielding effectiveness increases as the frequency increases, and the shielding effectiveness is negligible at high frequencies
In particular, it has been found that at frequencies in the range between 10 kHz to 10 GHz, and particularly at frequencies in the GHz region, with the aforementioned wire and cable type of shielding, it has not been possible to provide high attenuation characteristics, ie, at least 30 db, and preferable at least 40 to 60 db of attenuation, which is required in the art.
The goal of most shields is to provide as low an impedance path between both frame bodies on each end of the cable as possible. Optimally, a highly conductive, uniform shield material is desired. The braided shield provides shielded coverage proportional to the tightness of the braid. Thus, the tighter the braid, the higher the percent coverage of the shield. In terms of shielding ability of the cable, it is optimum to approach 100% coverage of the cable to maximize the shielding effectiveness. However, when shielding coverage by a braided cable reaches 90% and higher, the cable becomes much more rigid and inflexible. Cables using a foil wrapper help supplement braid. This increases the effective percent coverage. It is also difficult to maintain the integrity of a shield of high percent coverage braids over the life of the shield because of bending and movement of the cable. This causes separation of the braids and a degradation of shielding.
There are a number of problems associated with a metal/foil shield, however. First, on a cable assembly, it is usually a requirement that the braid make 360.degree. contact with the connector or premold. Because the braid will unravel and break if not cut properly, special processes must be used to obtain 360.degree. shielding at the connector. Second, complicated processes and expensive tooling are required to weave the small individual strands of wire that make up the metal braid over the cable. Third, due to the use of the metal braid and foil combination, cabling routing is limited in its achievable bend radius due to the metal braid and foil layers. Therefore, it would be desirable to provide a shielded bulk cable having 100% EMI and RFI shielding that does not require special processes to terminate at the connector, that doesn't require complicated processes and expensive tooling to manufacture, and that is more flexible than metal braid and foil cables to achieve greater bend radii.